1. Technical Field
The present invention is related generally to superconducting materials. More particularly, the present invention is related to new high temperature superconducting ceramic materials that contain thallium and to a process for the synthesis of said superconducting compositions.
2. State of the Art
Ceramic materials consist of sintered grains of metal oxide powders and, ideally, are highly dense and non-porous. Ceramic superconducting materials contain several constituent metal oxides and are produced by solid state reactions among the constituent oxides during high temperature sintering. The degree of sintering of the metal oxide powders is a function of the particle size, degree of agglomeration, and homogeneity of the mixture of the constituent metal oxide powders. Large size and agglomerated particles and inhomogeneous mixtures thereof result in a ceramic that is less sintered, inhomogeneous, porous, and brittle. Not only does the size of the metal oxide powder particles and homogeneity of the mixture of the powder particles affect the gross physical properties of the ceramic material, such as hardness and tensile strength, it affects the local chemical and electrical properties of such materials. Inhomogeneities in the ceramic that arise from incomplete mixing of the constituent metal oxide powders lead to unwanted phases and chemical gradients, which alter the conductive properties of the material. Ideally ceramic high temperature superconducting materials should be homogenous and non-porous.
High temperature superconducting ceramic materials are generally prepared by heating a ball-milled mixture of the powdered oxides of the component metal cations. This method of preparation, however has several drawbacks. The particles in the ball-milled mixed oxide powders are large and agglomerated and the ceramics produced therefrom are inhomogeneous (see, e.g., Morosin et al. (1988) Physica C vol. 152, pp. 413-23) and porous. Further, large size particle powders are not highly reactive so that calcining temperatures of greater than 800.degree. C. are necessary to drive the solid state reactions. High temperature calcining promotes further growth in particle size and partial sintering among the constituent particles, which further contributes to the high degree of the porosity of the ceramics.
Precipitation of insoluble salts of the metal cations from solution has been used as a means to achieve a more homogeneous mixture of the constitutent metal cations. Precipitation of powders from solution is designed to overcome some of the drawbacks inherent in the procedures in which metal oxide powders are mixed and has provided a means for eliminating problems that arise from inadequate mixing and agglomeration of the powders. In the Argonne precipitation method (Wang et al. Inorganic Chemistry vol. 26 pp. 1474-76 (1987)), which does not yield highly homogenous mixtures, a solution of mixed oxides is titrated with base which, depending upon the respective solubilities as a function of pH of the hydroxides of the constituent metal cations, leads to successive precipitation of the metal hydroxides. Because the metals precipitate sequentially the powders are not homogeneous. For example, a solution of mixed oxides of yttrium, barium, and copper is titrated with hydroxide, which first precipitates copper hydroxide followed by yttrium hydroxide. Because the hydroxide salts of barium are highly soluble, potassium carbonate is then added to precipitate barium carbonate. Because of the successive precipitation of the insoluble metal salts, the powders produced by this method are inhomogeneous. In addition, the powders are highly contaminated with the precipitating counter ions, such as potassium and chloride.
In order to overcome the problems inherent in the prior art methods we have developed a precipitation method for preparing fine grained chemically homogenous powders that can be used to produce homogeneous dense superconducting ceramics. This process is described in Bunker et al., U.S. Pat. No. 4,839,339, which is herein incorporated in its entirety by reference thereto. The precipitation method of Bunker et al., supra., provides controlled precipitation of insoluble salts by instantaneously mixing at a controlled pH two or more solutions, which respectively contain highly soluble components. A first solution is prepared that contains highly soluble salts of the parent metal cations, such as yttrium, barium, and copper nitrates at concentrations such that the ratios thereof match the stoichiometric ratios of said metals in the superconducting ceramic material. A selected volume of the first solution is instantaneously mixed with a selected volume of a second solution, that contains highly soluble salts of pyrolyzable precipitating anions, such as hydroxide and carbonate. The pH of the resulting mixture is, thus, controlled because it is between the pH of both solutions and is selected such that the concentration of each of the dissolved metal cations remaining in the solution after the addition of the precipitating anion solution is three orders of magnitude lower than in the starting solution. In addition, the counter ions in both solutions are chosen so that they are thermally decomposed during high temperature processing. This precipitation method produces finely grained homogenous powders that can be calcined and sintered to produce highly dense homogenous superconducting ceramics. This procedure is, however, only applicable to metal cations whose salts are highly soluble in the first solution and insoluble when the first solution is mixed with a second solution. Some metals, such as thallium, do not fit this criterion.
Superconducting systems that contain thallium, such as the Tl--Ca--Ba--Cu--O system, are well-known (see e.g., Morosin et al., supra., Hazen et al. (1988) Phys. Rev. Lttrs. vol. 60, pp. 1657-1660, and Kwak et al. (1988) Physica C Vol. 156, pp. 103-108). Superconducting materials in the Tl--Ca--Ba--Cu--O system have been prepared by methods that include the mixed-oxide milling procedures, discussed above (see, also, Morosin et al. supra., Voigt et al. (1988) Met. Res. Soc. Symp. Proc. vol. 99, pp. 635-638, and Hazen et al., supra.), and by deposition and annealing of the constituent metals on singlecrystal substrates to produce films thereof (see, e.g., Kwak, supra.). The methods for preparing bulk ceramics, however, suffer from the drawbacks, discussed above, that result from inhomogeneous and agglomerated mixtures of the constituent metals or metal oxides. In addition, the constituent thallium oxides are unstable and volatile. For example, Tl.sub.2 O.sub.3 decomposes at temperatures above 800.degree. C., and is very volatile. These properties pose a problem because ceramic superconductors are processed at temperatures above 800.degree. C. in order to drive the solid state reactions. Because of the volatility and instability of thallium oxides the annealing reactions can only be carried out for short times and excess thallium oxides must be added in order to drive the solid state reactions. It is, therefore, difficult to control the stoichiometry of the constituent metals in the final product. In addition, a multiplicity of undesired phases are formed (see, e.g., Hazen et al., supra.). The multiple phases are not only caused by the volatility of the thallium oxides but also by inhomogeneities and large size particles in the metal oxide powders or other starting material.
Although the general precipitation method of Bunker et al., supra., is useful for precipitating salts of cations of elements, such as calcium, barium, copper, and yttrium, which are highly soluble at low pH and insoluble at higher pH, this method cannot be used to prepare mixtures that contain cations, such as thallium in the +1 oxidation state, whose salts are highly soluble in aqueous solutions at all pH values. Almost all salts of thallium in the +1 oxidation state are highly soluble and do not precipitate at the pH ranges that precipitate other metal cation salts, such as the hydroxides or carbonates of barium, calcium, and copper. On the other hand salts of thallium in the +3 oxidation state are highly insoluble and cannot be dissolved in the first parent metal cation solution that contains soluble salts of the other metal cations. There is, thus, a need for an improved precipitation process for preparing powders for use in the preparation of highly dense homogenous superconducting materials that contain thallium, particularly for the preparation of homogeneous superconducting ceramics that contain thallium, calcium, barium, and copper.